To increase the magnetic force exerted by the electromagnet, the student should add more batteries in series at location 3.
Adding more batteries will increase the voltage across the wire, which will enhance the current flowing through it. According to Ampère's Law, a higher current will result in a greater magnetic field and, therefore, a stronger magnetic force from the electromagnet.
Increasing the length of wire at location 1 or reducing the number of wire turns at location 4 would not effectively increase the magnetic force as significantly. Opening the switch at location 2 would disable the electromagnet, reducing the magnetic force to zero.